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Chapter 6

Chapter 6. Introduction to Logic. Objectives. Explain AND logic. Describe OR logic. Explain NOT logic. Explore a truth table for specified logic. Determine if a PLC ladder rung is true or false under specified conditions. Hardwired Electrical Circuit Has Electrical Continuity.

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Chapter 6

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  1. Chapter 6 Introduction to Logic

  2. Objectives • Explain AND logic. • Describe OR logic. • Explain NOT logic. • Explore a truth table for specified logic. • Determine if a PLC ladder rung is true or false under specified conditions.

  3. Hardwired Electrical Circuit Has Electrical Continuity • Hardwired circuit has actual current flow. • This is called electrical continuity. • Current flows through switching device to directly control the load.

  4. Hardwired Relay Circuit

  5. Logical Continuity (1 of 3) • Hardwire relay circuits have electrical continuity or current flow. • PLCs do not operate on electrical continuity, but logical continuity.

  6. Logical Continuity (2 of 3) • A PLC ladder program resembles an electric schematic. • A PLC program is a set of instructions stored in memory. • PLC ladder symbols represent ladder program instructions.

  7. Logical Continuity (3 of 3) • PLC instructions examine the status of a bit at the associated address. • Bits are either a 1 or 0. • Instructions are either true or false. • PLCs have logical continuity.

  8. Signal Flow into and out of a PLC

  9. Input Signal Seen by Input Module • Signal from field device is wired to input module screw terminals. • These signals are called input signals or simply inputs. • Each input point has a unique identifier called an address.

  10. Input Module Isolates and Converts • Discrete input module determines if input is a valid ON signal. • The discrete input module converts the incoming electrical signal to +5V DC, or 0V DC. • An ON state will be converted to +5V DC. • An OFF state will be converted to 0V DC.

  11. Input Section to Back Plane • The high voltage input signal is isolated from the lower voltage microprocessor and supporting circuitry in the processor. • Signal is then transferred to the chassis back plane and sent to processor memory for storage.

  12. Processor and Input Status File • Field input signals are transferred from the input module by way of the back plane for storage in the processor’s input status file. • There is one storage location in the status table for the ON or OFF status of each input device. • The ON signal is stored as a 1; the OFF as a 0.

  13. Processor Examines Status Table Bit • When running, the processor solves ladder instructions by examining the ON or OFF status of the input device by examining the associated memory location for a 1 or 0.

  14. Ladder Program Solved • Ladder instructions have rules regarding when they are true or false. • Instructions are made true or false as a result of the examined bit. • If there is a path of true input instructions to the output instruction, the rung will be true.

  15. Output Update • When all ladder rungs have been solved, the output status table data is sent to the respective output modules one word at a time. • When a rung is solved, the output status table is updated.

  16. Combining and Solving Ladder Instructions • Three basic types of logic: • And • Or • And / Or (combinational)

  17. Series Logic • When two switches are wired in series, both switches must be on before power will flow. • Switch 1 AND switch 2 must all pass power before light 1 will turn on.

  18. Conventional Series Circuit

  19. Switch 1 AND Switch 2 Need to Be Closed to Energize L-1

  20. Truth Table • A truth table shows all possible input conditions and the expected output conditions. • The following slide shows a truth table for AND logic.

  21. Truth Table for AND Logic

  22. PLC Instructions in Series • PLC instructions work the same basic way as hardwired field devices. • Instructions in series are considered ANDed together, or AND logic. • Switch 1 AND switch 2 must be on for L-1 to come on.

  23. PLC Representation of a Rung

  24. Truth Table for Two-Input AND Logic

  25. Three-Input Series Circuit

  26. Three-Input PLC Ladder Rung

  27. Three-Input AND Logic Truth Table

  28. Parallel Is also OR Logic • Inputs in parallel are said to be ORed. • Switch 1 is ORed with switch 2. • If switch 1 is closed OR switch 2 is closed, light 1 will turn on. • If both SW1 and SW2 are closed, light 1 will turn on.

  29. Conventional Parallel Circuit

  30. Programmable Controller Ladder Diagram

  31. Two-input OR Truth Table

  32. Truth Table for NOT Logic

  33. Conventional Ladder Diagram

  34. PLC Representation of N.O. and N.C. Instructions • All PLCs use the standard normally open (N.O.) and normally closed (N.C.) instructions. • Different manufacturers use different names for the instructions.

  35. Normally Open and Normally Closed Instruction Identification

  36. Combination AND-OR Logic • Input devices can be combined in series (ANDed) and also in parallel (ORed) on a PLC ladder rung. • Instructions are programmed to provide the proper input conditions for your application. • This is called combination AND-OR logic.

  37. Combination AND-OR Logic

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